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DTC codes / Mazda codes / P1152

P1152 · MazdaHO2S Bank 2 Sensor 1 Signal Above 0.45v AF Ratio Too Rich

P1152 on Mazda vehicles means the powertrain control module (PCM) has seen the Bank 2 upstream heated oxygen sensor (HO2S) signal stay above 0.45 volts for longer than expected, which tells the PCM the exhaust is running too rich on that bank. The sensor itself is reporting a rich condition, so the PCM is flagging the signal as out of range rather than commanding more fuel.

SystemPowertrain
Code typeMazda specific
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering

Symptoms you may notice

  • Often no symptoms besides the warning light, especially if the rich condition is mild or intermittent.
  • A noticeable drop in fuel economy as the engine consumes more fuel than the PCM expects.
  • Black sooty exhaust smoke or a strong fuel smell from the tailpipe when the mixture is heavily rich.
  • Rough idle, hesitation, or stumbling acceleration because the Bank 2 cylinders are over-fueled.
  • A sulfur or rotten-egg smell from the catalytic converter if the rich condition has been present for a while.
  • Possible misfire codes or a flashing check engine light if raw fuel reaches the converter and overheats it.

Common causes, most likely first

  1. A failed or contaminated Bank 2 upstream heated oxygen sensor that is stuck reading rich.
  2. A leaking fuel injector on Bank 2 delivering excess fuel into one cylinder.
  3. High fuel pressure from a faulty fuel pressure regulator or a restricted fuel return line.
  4. A stuck-open or leaking purge solenoid allowing fuel vapor to flood the intake.
  5. An intake or exhaust leak on Bank 2 that skews the sensor's oxygen reading.
  6. A wiring fault in the HO2S signal circuit, such as a short to voltage or a chafed harness.
  7. A PCM calibration issue or a failing PCM driver for that sensor circuit, though this is rare.

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data for P1152, then check for related misfire or fuel trim codes before touching any parts.
  2. Inspect the Bank 2 upstream HO2S wiring and connector for chafing, corrosion, or a short to voltage, and repair as needed.
  3. Use a scan tool to watch Bank 2 upstream HO2S voltage and short-term fuel trim while revving the engine; a sensor stuck above 0.45 volts confirms a rich signal.
  4. Check fuel pressure with a gauge and compare it to the Mazda specification for the engine, looking for a regulator or return-line fault.
  5. Perform an injector balance or leak-down test on the Bank 2 injectors to find a leaking or stuck-open injector.
  6. Smoke-test the intake and exhaust on Bank 2 to rule out leaks that can skew the oxygen sensor reading.
  7. If the sensor signal is confirmed stuck rich and all other causes are ruled out, replace the Bank 2 upstream HO2S with the correct Mazda-specified part and clear the code.

What a rich Bank 2 signal really tells you

  • A P1152 does not automatically mean the oxygen sensor is bad; a leaking injector or high fuel pressure can hold the sensor signal rich and fool a parts-swapping diagnosis.
  • Mazda's upstream HO2S on Bank 2 is often the sensor that gets contaminated by coolant or oil from a failing head gasket or valve cover leak, so check for contamination before condemning the sensor.
  • A stuck-open purge solenoid can flood the intake with fuel vapor and set a rich code on one bank, especially after refueling, and it is frequently missed because the purge system is not checked.
  • The 0.45-volt threshold is a midpoint reference, so a sensor that is merely slow or lazy can cross it intermittently and set P1152 without ever failing a simple resistance test.
  • If the code returns immediately after a new sensor is installed, check the Bank 2 fuel trim and injector pulse width with a scan tool; the PCM may be adding fuel because of a mechanical fault, not a sensor fault.

Can I keep driving with P1152?

You can drive a short distance to a repair shop if the engine runs smoothly and the check engine light is steady, but avoid hard acceleration and long trips because a rich condition can overheat and damage the catalytic converter. If the light flashes or the engine misfires, stop driving and have the vehicle towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Mazda vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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